The present invention relates to integrated circuit devices, and in particular high voltage transistors, power MOSFETs, IGBTs, thyristors, MCTs, and the like (“power devices”). Merely by way of example, the present invention is illustrated with an insulated gate bipolar transistor (IGBT) fabrication method and structure.
High voltage transistors such as conventional insulated gate bipolar transistors and the like (“conventional IGBTs”), are fabricated by conventional semiconductor processing techniques on a single crystalline semiconductor substrate, such as a silicon wafer. Conventional semiconductor processing techniques include doping and implanting, lithography, diffusion, chemical vapor deposition (CVD), wet and dry etching, sputtering, epitaxy, and oxidizing. A complex sequence of these processing techniques is often required to produce the conventional IGBT having a high breakdown voltage.
FIG. 1 illustrates a circuit diagram for the conventional IGBT 10. The conventional IGBT includes a gate terminal (G) 11, a drain terminal (D) 13, and a source terminal (S) 15. As shown, a positive voltage potential exists between the drain terminal 13 and the source terminal 15. No switching voltage exists at the gate terminal when the device is in an off-state, and no electrical current passes from the drain terminal 13 to the source terminal 15 in the off-state. The conventional IGBT turns “on” to an on-state when a switching voltage is applied to the gate terminal 11. Current passes from the drain terminal 13 to the source terminal 15 in the on-state.
The conventional IGBT includes a voltage blocking rating only in one direction. In particular, the conventional IGBT provides a “forward blocking” mode to block electrical current therethrough. In the forward blocking mode, the gate is in an off-state, high voltage appears on the drain terminal 13, and low voltage appears on the source terminal 15. Substantially no electrical current flows through the conventional IGBT in the forward blocking mode. It should be noted the forward blocking mode corresponds to the same biasing conditions on the drain terminal and the source terminal as the forward conduction mode, when the device is turned-on.
One limitation with the conventional IGBT 20 is that device break down often occurs when relatively low voltage is applied to the device in a reverse blocking mode configuration as illustrated by FIG. 2. In the reverse blocking mode, a positive voltage potential is applied to the source terminal while the gate terminal is in an off-state. The relatively low voltage such as 30-50 volts applied to the source terminal 15, relative to the drain terminal 13, causes uncontrolled conduction of electrical current through the device even though the gate is in the off-state as illustrated by FIG. 3.
FIG. 3 illustrates IDS (a current from drain to source) as a function of VDS (a potential difference from drain to source) for a conventional IGBT device having a breakdown voltage at about 1,800 volts. The conventional IGBT device breaks down causing an uncontrolled conduction of current through the device at about 1,800 volts in the forward blocking mode and at about −35 volts in the reverse blocking mode. Accordingly, the application of conventional IGBTs is generally limited to direct current (DC) configurations operating in the forward conduction mode.
However, it is often desirable to use an IGBT for alternating current (AC) applications, which subject the IGBT to both positive and negative voltage potentials at source and drain terminals. The conventional IGBT, unfortunately, cannot effectively block a high negative voltage potential because of its limited reverse blocking rating.